Unit 4 Session 1 Standard (Click for link to problem statements)

Understand what the interviewer is asking for by using test cases and questions about the problem.

• Q: What is the structure of the input?

  • A: The input is a list of strings, where each string is either "add" or "remove".

• Q: What is the output?

  • A: The output is a boolean value: True if the actions are balanced, False otherwise.

• Q: What does it mean for the actions to be balanced?

  • A: Every "add" action must have a corresponding "remove" action, and no "remove" action should occur before a matching "add".

• Q: What should be returned if the list is empty?

  • A: True, since an empty sequence is trivially balanced.

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Use a stack to track "add" actions. For each "remove" action, check if there is a corresponding "add" on the stack. If the stack is empty when a "remove" is encountered, or if any "add" is left unmatched, the sequence is unbalanced.

1) Initialize an empty list called `stack` to simulate the stack behavior. 2) Iterate through each `action` in `actions`: a) If `action` is `"add"`, push it onto the stack. b) If `action` is `"remove"`, check if the stack is empty: i) If empty, return `False`. ii) If not empty, pop the top of the stack. 3) After the loop, return `True` if the stack is empty, otherwise return `False`. **⚠️ Common Mistakes** - Forgetting to return `False` if a `"remove"` action occurs with an empty stack. - Not ensuring the stack is empty at the end to confirm all `"add"` actions were matched.

def validate_nft_actions(actions): stack = [] for action in actions: if action == "add": stack.append(action) elif action == "remove": if not stack: return False stack.pop() return len(stack) == 0